2 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
4 use rustc_ast::util::{classify, parser};
5 use rustc_ast::{ExprKind, StmtKind};
6 use rustc_errors::{fluent, pluralize, Applicability, MultiSpan};
8 use rustc_hir::def::{DefKind, Res};
9 use rustc_hir::def_id::DefId;
10 use rustc_middle::ty::adjustment;
11 use rustc_middle::ty::{self, Ty};
12 use rustc_span::symbol::Symbol;
13 use rustc_span::symbol::{kw, sym};
14 use rustc_span::{BytePos, Span};
17 /// The `unused_must_use` lint detects unused result of a type flagged as
23 /// fn returns_result() -> Result<(), ()> {
36 /// The `#[must_use]` attribute is an indicator that it is a mistake to
37 /// ignore the value. See [the reference] for more details.
39 /// [the reference]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
42 "unused result of a type flagged as `#[must_use]`",
43 report_in_external_macro
47 /// The `unused_results` lint checks for the unused result of an
48 /// expression in a statement.
52 /// ```rust,compile_fail
53 /// #![deny(unused_results)]
54 /// fn foo<T>() -> T { panic!() }
65 /// Ignoring the return value of a function may indicate a mistake. In
66 /// cases were it is almost certain that the result should be used, it is
67 /// recommended to annotate the function with the [`must_use` attribute].
68 /// Failure to use such a return value will trigger the [`unused_must_use`
69 /// lint] which is warn-by-default. The `unused_results` lint is
70 /// essentially the same, but triggers for *all* return values.
72 /// This lint is "allow" by default because it can be noisy, and may not be
73 /// an actual problem. For example, calling the `remove` method of a `Vec`
74 /// or `HashMap` returns the previous value, which you may not care about.
75 /// Using this lint would require explicitly ignoring or discarding such
78 /// [`must_use` attribute]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
79 /// [`unused_must_use` lint]: warn-by-default.html#unused-must-use
82 "unused result of an expression in a statement"
85 declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);
87 impl<'tcx> LateLintPass<'tcx> for UnusedResults {
88 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
89 let expr = match s.kind {
90 hir::StmtKind::Semi(ref expr) => &**expr,
94 if let hir::ExprKind::Ret(..) = expr.kind {
98 let ty = cx.typeck_results().expr_ty(&expr);
99 let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
101 let mut fn_warned = false;
102 let mut op_warned = false;
103 let maybe_def_id = match expr.kind {
104 hir::ExprKind::Call(ref callee, _) => {
106 hir::ExprKind::Path(ref qpath) => {
107 match cx.qpath_res(qpath, callee.hir_id) {
108 Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => Some(def_id),
109 // `Res::Local` if it was a closure, for which we
110 // do not currently support must-use linting
117 hir::ExprKind::MethodCall(..) => cx.typeck_results().type_dependent_def_id(expr.hir_id),
120 if let Some(def_id) = maybe_def_id {
121 fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
122 } else if type_permits_lack_of_use {
123 // We don't warn about unused unit or uninhabited types.
124 // (See https://github.com/rust-lang/rust/issues/43806 for details.)
128 let must_use_op = match expr.kind {
129 // Hardcoding operators here seemed more expedient than the
130 // refactoring that would be needed to look up the `#[must_use]`
131 // attribute which does exist on the comparison trait methods
132 hir::ExprKind::Binary(bin_op, ..) => match bin_op.node {
138 | hir::BinOpKind::Gt => Some("comparison"),
140 | hir::BinOpKind::Sub
141 | hir::BinOpKind::Div
142 | hir::BinOpKind::Mul
143 | hir::BinOpKind::Rem => Some("arithmetic operation"),
144 hir::BinOpKind::And | hir::BinOpKind::Or => Some("logical operation"),
145 hir::BinOpKind::BitXor
146 | hir::BinOpKind::BitAnd
147 | hir::BinOpKind::BitOr
148 | hir::BinOpKind::Shl
149 | hir::BinOpKind::Shr => Some("bitwise operation"),
151 hir::ExprKind::AddrOf(..) => Some("borrow"),
152 hir::ExprKind::Unary(..) => Some("unary operation"),
156 if let Some(must_use_op) = must_use_op {
157 cx.struct_span_lint(UNUSED_MUST_USE, expr.span, fluent::lint::unused_op, |lint| {
158 lint.set_arg("op", must_use_op)
159 .span_label(expr.span, fluent::lint::label)
160 .span_suggestion_verbose(
161 expr.span.shrink_to_lo(),
162 fluent::lint::suggestion,
164 Applicability::MachineApplicable,
170 if !(type_permits_lack_of_use || fn_warned || op_warned) {
171 cx.struct_span_lint(UNUSED_RESULTS, s.span, fluent::lint::unused_result, |lint| {
172 lint.set_arg("ty", ty)
176 // Returns whether an error has been emitted (and thus another does not need to be later).
177 fn check_must_use_ty<'tcx>(
178 cx: &LateContext<'tcx>,
180 expr: &hir::Expr<'_>,
187 || cx.tcx.is_ty_uninhabited_from(
188 cx.tcx.parent_module(expr.hir_id).to_def_id(),
196 let plural_suffix = pluralize!(plural_len);
199 ty::Adt(..) if ty.is_box() => {
200 let boxed_ty = ty.boxed_ty();
201 let descr_pre = &format!("{}boxed ", descr_pre);
202 check_must_use_ty(cx, boxed_ty, expr, span, descr_pre, descr_post, plural_len)
204 ty::Adt(def, _) => check_must_use_def(cx, def.did(), span, descr_pre, descr_post),
205 ty::Opaque(def, _) => {
206 let mut has_emitted = false;
207 for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
208 // We only look at the `DefId`, so it is safe to skip the binder here.
209 if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
210 predicate.kind().skip_binder()
212 let def_id = poly_trait_predicate.trait_ref.def_id;
214 &format!("{}implementer{} of ", descr_pre, plural_suffix,);
215 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
223 ty::Dynamic(binder, _, _) => {
224 let mut has_emitted = false;
225 for predicate in binder.iter() {
226 if let ty::ExistentialPredicate::Trait(ref trait_ref) =
227 predicate.skip_binder()
229 let def_id = trait_ref.def_id;
231 &format!(" trait object{}{}", plural_suffix, descr_post,);
232 if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
240 ty::Tuple(ref tys) => {
241 let mut has_emitted = false;
242 let comps = if let hir::ExprKind::Tup(comps) = expr.kind {
243 debug_assert_eq!(comps.len(), tys.len());
248 for (i, ty) in tys.iter().enumerate() {
249 let descr_post = &format!(" in tuple element {}", i);
250 let e = comps.get(i).unwrap_or(expr);
252 if check_must_use_ty(cx, ty, e, span, descr_pre, descr_post, plural_len) {
258 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
259 // If the array is empty we don't lint, to avoid false positives
260 Some(0) | None => false,
261 // If the array is definitely non-empty, we can do `#[must_use]` checking.
263 let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix,);
264 check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
271 fluent::lint::unused_closure,
273 // FIXME(davidtwco): this isn't properly translatable because of the
275 lint.set_arg("count", plural_len)
276 .set_arg("pre", descr_pre)
277 .set_arg("post", descr_post)
278 .note(fluent::lint::note)
283 ty::Generator(..) => {
287 fluent::lint::unused_generator,
289 // FIXME(davidtwco): this isn't properly translatable because of the
291 lint.set_arg("count", plural_len)
292 .set_arg("pre", descr_pre)
293 .set_arg("post", descr_post)
294 .note(fluent::lint::note)
303 // Returns whether an error has been emitted (and thus another does not need to be later).
304 // FIXME: Args desc_{pre,post}_path could be made lazy by taking Fn() -> &str, but this
305 // would make calling it a big awkward. Could also take String (so args are moved), but
306 // this would still require a copy into the format string, which would only be executed
308 fn check_must_use_def(
309 cx: &LateContext<'_>,
312 descr_pre_path: &str,
313 descr_post_path: &str,
315 if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
316 cx.struct_span_lint(UNUSED_MUST_USE, span, fluent::lint::unused_def, |lint| {
317 // FIXME(davidtwco): this isn't properly translatable because of the pre/post
319 lint.set_arg("pre", descr_pre_path);
320 lint.set_arg("post", descr_post_path);
321 lint.set_arg("def", cx.tcx.def_path_str(def_id));
322 // check for #[must_use = "..."]
323 if let Some(note) = attr.value_str() {
324 lint.note(note.as_str());
337 /// The `path_statements` lint detects path statements with no effect.
351 /// It is usually a mistake to have a statement that has no effect.
354 "path statements with no effect"
357 declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);
359 impl<'tcx> LateLintPass<'tcx> for PathStatements {
360 fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
361 if let hir::StmtKind::Semi(expr) = s.kind {
362 if let hir::ExprKind::Path(_) = expr.kind {
363 let ty = cx.typeck_results().expr_ty(expr);
364 if ty.needs_drop(cx.tcx, cx.param_env) {
368 fluent::lint::path_statement_drop,
370 if let Ok(snippet) = cx.sess().source_map().span_to_snippet(expr.span) {
371 lint.span_suggestion(
373 fluent::lint::suggestion,
374 format!("drop({});", snippet),
375 Applicability::MachineApplicable,
378 lint.span_help(s.span, fluent::lint::suggestion);
387 fluent::lint::path_statement_no_effect,
396 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
397 enum UnusedDelimsCtx {
401 AssignedValueLetElse,
414 impl From<UnusedDelimsCtx> for &'static str {
415 fn from(ctx: UnusedDelimsCtx) -> &'static str {
417 UnusedDelimsCtx::FunctionArg => "function argument",
418 UnusedDelimsCtx::MethodArg => "method argument",
419 UnusedDelimsCtx::AssignedValue | UnusedDelimsCtx::AssignedValueLetElse => {
422 UnusedDelimsCtx::IfCond => "`if` condition",
423 UnusedDelimsCtx::WhileCond => "`while` condition",
424 UnusedDelimsCtx::ForIterExpr => "`for` iterator expression",
425 UnusedDelimsCtx::MatchScrutineeExpr => "`match` scrutinee expression",
426 UnusedDelimsCtx::ReturnValue => "`return` value",
427 UnusedDelimsCtx::BlockRetValue => "block return value",
428 UnusedDelimsCtx::LetScrutineeExpr => "`let` scrutinee expression",
429 UnusedDelimsCtx::ArrayLenExpr | UnusedDelimsCtx::AnonConst => "const expression",
430 UnusedDelimsCtx::MatchArmExpr => "match arm expression",
435 /// Used by both `UnusedParens` and `UnusedBraces` to prevent code duplication.
436 trait UnusedDelimLint {
437 const DELIM_STR: &'static str;
439 /// Due to `ref` pattern, there can be a difference between using
440 /// `{ expr }` and `expr` in pattern-matching contexts. This means
441 /// that we should only lint `unused_parens` and not `unused_braces`
446 /// let ref b = { a }; // We actually borrow a copy of `a` here.
447 /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
448 /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
450 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool;
452 // this cannot be a constant is it refers to a static.
453 fn lint(&self) -> &'static Lint;
455 fn check_unused_delims_expr(
457 cx: &EarlyContext<'_>,
459 ctx: UnusedDelimsCtx,
460 followed_by_block: bool,
461 left_pos: Option<BytePos>,
462 right_pos: Option<BytePos>,
465 fn is_expr_delims_necessary(
467 followed_by_block: bool,
468 followed_by_else: bool,
470 if followed_by_else {
472 ast::ExprKind::Binary(op, ..) if op.node.lazy() => return true,
473 _ if classify::expr_trailing_brace(inner).is_some() => return true,
478 // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
479 let lhs_needs_parens = {
480 let mut innermost = inner;
482 innermost = match &innermost.kind {
483 ExprKind::Binary(_, lhs, _rhs) => lhs,
484 ExprKind::Call(fn_, _params) => fn_,
485 ExprKind::Cast(expr, _ty) => expr,
486 ExprKind::Type(expr, _ty) => expr,
487 ExprKind::Index(base, _subscript) => base,
490 if !classify::expr_requires_semi_to_be_stmt(innermost) {
497 || (followed_by_block
498 && match &inner.kind {
499 ExprKind::Ret(_) | ExprKind::Break(..) | ExprKind::Yield(..) => true,
500 ExprKind::Range(_lhs, Some(rhs), _limits) => {
501 matches!(rhs.kind, ExprKind::Block(..))
503 _ => parser::contains_exterior_struct_lit(&inner),
507 fn emit_unused_delims_expr(
509 cx: &EarlyContext<'_>,
511 ctx: UnusedDelimsCtx,
512 left_pos: Option<BytePos>,
513 right_pos: Option<BytePos>,
515 let spans = match value.kind {
516 ast::ExprKind::Block(ref block, None) if block.stmts.len() > 0 => {
517 let start = block.stmts[0].span;
518 let end = block.stmts[block.stmts.len() - 1].span;
519 if let Some(start) = start.find_ancestor_inside(value.span)
520 && let Some(end) = end.find_ancestor_inside(value.span)
523 value.span.with_hi(start.lo()),
524 value.span.with_lo(end.hi()),
530 ast::ExprKind::Paren(ref expr) => {
531 let expr_span = expr.span.find_ancestor_inside(value.span);
532 if let Some(expr_span) = expr_span {
533 Some((value.span.with_hi(expr_span.lo()), value.span.with_lo(expr_span.hi())))
541 left_pos.map_or(false, |s| s >= value.span.lo()),
542 right_pos.map_or(false, |s| s <= value.span.hi()),
544 self.emit_unused_delims(cx, value.span, spans, ctx.into(), keep_space);
547 fn emit_unused_delims(
549 cx: &EarlyContext<'_>,
551 spans: Option<(Span, Span)>,
553 keep_space: (bool, bool),
555 let primary_span = if let Some((lo, hi)) = spans {
556 MultiSpan::from(vec![lo, hi])
558 MultiSpan::from(value_span)
560 cx.struct_span_lint(self.lint(), primary_span, fluent::lint::unused_delim, |lint| {
561 lint.set_arg("delim", Self::DELIM_STR);
562 lint.set_arg("item", msg);
563 if let Some((lo, hi)) = spans {
564 let replacement = vec![
565 (lo, if keep_space.0 { " ".into() } else { "".into() }),
566 (hi, if keep_space.1 { " ".into() } else { "".into() }),
568 lint.multipart_suggestion(
569 fluent::lint::suggestion,
571 Applicability::MachineApplicable,
578 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
579 use rustc_ast::ExprKind::*;
580 let (value, ctx, followed_by_block, left_pos, right_pos) = match e.kind {
581 // Do not lint `unused_braces` in `if let` expressions.
582 If(ref cond, ref block, _)
583 if !matches!(cond.kind, Let(_, _, _))
584 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
586 let left = e.span.lo() + rustc_span::BytePos(2);
587 let right = block.span.lo();
588 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
591 // Do not lint `unused_braces` in `while let` expressions.
592 While(ref cond, ref block, ..)
593 if !matches!(cond.kind, Let(_, _, _))
594 || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
596 let left = e.span.lo() + rustc_span::BytePos(5);
597 let right = block.span.lo();
598 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
601 ForLoop(_, ref cond, ref block, ..) => {
602 (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()))
605 Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
606 let left = e.span.lo() + rustc_span::BytePos(5);
607 (head, UnusedDelimsCtx::MatchScrutineeExpr, true, Some(left), None)
610 Ret(Some(ref value)) => {
611 let left = e.span.lo() + rustc_span::BytePos(3);
612 (value, UnusedDelimsCtx::ReturnValue, false, Some(left), None)
615 Assign(_, ref value, _) | AssignOp(.., ref value) => {
616 (value, UnusedDelimsCtx::AssignedValue, false, None, None)
618 // either function/method call, or something this lint doesn't care about
619 ref call_or_other => {
620 let (args_to_check, ctx) = match *call_or_other {
621 Call(_, ref args) => (&args[..], UnusedDelimsCtx::FunctionArg),
622 MethodCall(_, _, ref args, _) => (&args[..], UnusedDelimsCtx::MethodArg),
623 // actual catch-all arm
628 // Don't lint if this is a nested macro expansion: otherwise, the lint could
629 // trigger in situations that macro authors shouldn't have to care about, e.g.,
630 // when a parenthesized token tree matched in one macro expansion is matched as
631 // an expression in another and used as a fn/method argument (Issue #47775)
632 if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
635 for arg in args_to_check {
636 self.check_unused_delims_expr(cx, arg, ctx, false, None, None);
641 self.check_unused_delims_expr(cx, &value, ctx, followed_by_block, left_pos, right_pos);
644 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
646 StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
647 if let Some((init, els)) = local.kind.init_else_opt() {
648 let ctx = match els {
649 None => UnusedDelimsCtx::AssignedValue,
650 Some(_) => UnusedDelimsCtx::AssignedValueLetElse,
652 self.check_unused_delims_expr(cx, init, ctx, false, None, None);
655 StmtKind::Expr(ref expr) => {
656 self.check_unused_delims_expr(
659 UnusedDelimsCtx::BlockRetValue,
669 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
670 use ast::ItemKind::*;
672 if let Const(.., Some(expr)) | Static(.., Some(expr)) = &item.kind {
673 self.check_unused_delims_expr(
676 UnusedDelimsCtx::AssignedValue,
686 /// The `unused_parens` lint detects `if`, `match`, `while` and `return`
687 /// with parentheses; they do not need them.
699 /// The parentheses are not needed, and should be removed. This is the
700 /// preferred style for writing these expressions.
701 pub(super) UNUSED_PARENS,
703 "`if`, `match`, `while` and `return` do not need parentheses"
706 declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);
708 impl UnusedDelimLint for UnusedParens {
709 const DELIM_STR: &'static str = "parentheses";
711 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = true;
713 fn lint(&self) -> &'static Lint {
717 fn check_unused_delims_expr(
719 cx: &EarlyContext<'_>,
721 ctx: UnusedDelimsCtx,
722 followed_by_block: bool,
723 left_pos: Option<BytePos>,
724 right_pos: Option<BytePos>,
727 ast::ExprKind::Paren(ref inner) => {
728 let followed_by_else = ctx == UnusedDelimsCtx::AssignedValueLetElse;
729 if !Self::is_expr_delims_necessary(inner, followed_by_block, followed_by_else)
730 && value.attrs.is_empty()
731 && !value.span.from_expansion()
732 && (ctx != UnusedDelimsCtx::LetScrutineeExpr
733 || !matches!(inner.kind, ast::ExprKind::Binary(
734 rustc_span::source_map::Spanned { node, .. },
739 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
742 ast::ExprKind::Let(_, ref expr, _) => {
743 self.check_unused_delims_expr(
746 UnusedDelimsCtx::LetScrutineeExpr,
758 fn check_unused_parens_pat(
760 cx: &EarlyContext<'_>,
765 use ast::{BindingAnnotation, PatKind};
767 if let PatKind::Paren(inner) = &value.kind {
769 // The lint visitor will visit each subpattern of `p`. We do not want to lint
770 // any range pattern no matter where it occurs in the pattern. For something like
771 // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
772 // that if there are unnecessary parens they serve a purpose of readability.
773 PatKind::Range(..) => return,
774 // Avoid `p0 | .. | pn` if we should.
775 PatKind::Or(..) if avoid_or => return,
776 // Avoid `mut x` and `mut x @ p` if we should:
777 PatKind::Ident(BindingAnnotation::MUT, ..) if avoid_mut => {
780 // Otherwise proceed with linting.
783 let spans = if let Some(inner) = inner.span.find_ancestor_inside(value.span) {
784 Some((value.span.with_hi(inner.lo()), value.span.with_lo(inner.hi())))
788 self.emit_unused_delims(cx, value.span, spans, "pattern", (false, false));
793 impl EarlyLintPass for UnusedParens {
794 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
796 ExprKind::Let(ref pat, _, _) | ExprKind::ForLoop(ref pat, ..) => {
797 self.check_unused_parens_pat(cx, pat, false, false);
799 // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
800 // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
801 // want to complain about things like `if let 42 = (42)`.
802 ExprKind::If(ref cond, ref block, ref else_)
803 if matches!(cond.peel_parens().kind, ExprKind::Let(..)) =>
805 self.check_unused_delims_expr(
808 UnusedDelimsCtx::LetScrutineeExpr,
813 for stmt in &block.stmts {
814 <Self as UnusedDelimLint>::check_stmt(self, cx, stmt);
816 if let Some(e) = else_ {
817 <Self as UnusedDelimLint>::check_expr(self, cx, e);
821 ExprKind::Match(ref _expr, ref arm) => {
823 self.check_unused_delims_expr(
826 UnusedDelimsCtx::MatchArmExpr,
836 <Self as UnusedDelimLint>::check_expr(self, cx, e)
839 fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
840 use ast::{Mutability, PatKind::*};
842 // Do not lint on `(..)` as that will result in the other arms being useless.
844 // The other cases do not contain sub-patterns.
845 | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
846 // These are list-like patterns; parens can always be removed.
847 TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
848 self.check_unused_parens_pat(cx, p, false, false);
850 Struct(_, _, fps, _) => for f in fps {
851 self.check_unused_parens_pat(cx, &f.pat, false, false);
853 // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
854 Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
855 // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
856 // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
857 Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not),
861 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
862 if let StmtKind::Local(ref local) = s.kind {
863 self.check_unused_parens_pat(cx, &local.pat, true, false);
866 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
869 fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
870 self.check_unused_parens_pat(cx, ¶m.pat, true, false);
873 fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
874 self.check_unused_parens_pat(cx, &arm.pat, false, false);
877 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
878 if let ast::TyKind::Paren(r) = &ty.kind {
880 ast::TyKind::TraitObject(..) => {}
881 ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
882 ast::TyKind::Array(_, len) => {
883 self.check_unused_delims_expr(
886 UnusedDelimsCtx::ArrayLenExpr,
893 let spans = if let Some(r) = r.span.find_ancestor_inside(ty.span) {
894 Some((ty.span.with_hi(r.lo()), ty.span.with_lo(r.hi())))
898 self.emit_unused_delims(cx, ty.span, spans, "type", (false, false));
904 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
905 <Self as UnusedDelimLint>::check_item(self, cx, item)
910 /// The `unused_braces` lint detects unnecessary braces around an
925 /// The braces are not needed, and should be removed. This is the
926 /// preferred style for writing these expressions.
927 pub(super) UNUSED_BRACES,
929 "unnecessary braces around an expression"
932 declare_lint_pass!(UnusedBraces => [UNUSED_BRACES]);
934 impl UnusedDelimLint for UnusedBraces {
935 const DELIM_STR: &'static str = "braces";
937 const LINT_EXPR_IN_PATTERN_MATCHING_CTX: bool = false;
939 fn lint(&self) -> &'static Lint {
943 fn check_unused_delims_expr(
945 cx: &EarlyContext<'_>,
947 ctx: UnusedDelimsCtx,
948 followed_by_block: bool,
949 left_pos: Option<BytePos>,
950 right_pos: Option<BytePos>,
953 ast::ExprKind::Block(ref inner, None)
954 if inner.rules == ast::BlockCheckMode::Default =>
956 // emit a warning under the following conditions:
958 // - the block does not have a label
959 // - the block is not `unsafe`
960 // - the block contains exactly one expression (do not lint `{ expr; }`)
961 // - `followed_by_block` is true and the internal expr may contain a `{`
962 // - the block is not multiline (do not lint multiline match arms)
966 // somewhat_long_expression
971 // - the block has no attribute and was not created inside a macro
972 // - if the block is an `anon_const`, the inner expr must be a literal
973 // (do not lint `struct A<const N: usize>; let _: A<{ 2 + 3 }>;`)
975 // FIXME(const_generics): handle paths when #67075 is fixed.
976 if let [stmt] = inner.stmts.as_slice() {
977 if let ast::StmtKind::Expr(ref expr) = stmt.kind {
978 if !Self::is_expr_delims_necessary(expr, followed_by_block, false)
979 && (ctx != UnusedDelimsCtx::AnonConst
980 || matches!(expr.kind, ast::ExprKind::Lit(_)))
981 && !cx.sess().source_map().is_multiline(value.span)
982 && value.attrs.is_empty()
983 && !value.span.from_expansion()
985 self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
990 ast::ExprKind::Let(_, ref expr, _) => {
991 self.check_unused_delims_expr(
994 UnusedDelimsCtx::LetScrutineeExpr,
1005 impl EarlyLintPass for UnusedBraces {
1006 fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
1007 <Self as UnusedDelimLint>::check_stmt(self, cx, s)
1010 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
1011 <Self as UnusedDelimLint>::check_expr(self, cx, e);
1013 if let ExprKind::Repeat(_, ref anon_const) = e.kind {
1014 self.check_unused_delims_expr(
1017 UnusedDelimsCtx::AnonConst,
1025 fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
1026 if let ast::GenericArg::Const(ct) = arg {
1027 self.check_unused_delims_expr(
1030 UnusedDelimsCtx::AnonConst,
1038 fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
1039 if let Some(anon_const) = &v.disr_expr {
1040 self.check_unused_delims_expr(
1043 UnusedDelimsCtx::AnonConst,
1051 fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
1053 ast::TyKind::Array(_, ref len) => {
1054 self.check_unused_delims_expr(
1057 UnusedDelimsCtx::ArrayLenExpr,
1064 ast::TyKind::Typeof(ref anon_const) => {
1065 self.check_unused_delims_expr(
1068 UnusedDelimsCtx::AnonConst,
1079 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1080 <Self as UnusedDelimLint>::check_item(self, cx, item)
1085 /// The `unused_import_braces` lint catches unnecessary braces around an
1090 /// ```rust,compile_fail
1091 /// #![deny(unused_import_braces)]
1104 /// If there is only a single item, then remove the braces (`use test::A;`
1107 /// This lint is "allow" by default because it is only enforcing a
1108 /// stylistic choice.
1109 UNUSED_IMPORT_BRACES,
1111 "unnecessary braces around an imported item"
1114 declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);
1116 impl UnusedImportBraces {
1117 fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
1118 if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
1119 // Recursively check nested UseTrees
1120 for &(ref tree, _) in items {
1121 self.check_use_tree(cx, tree, item);
1124 // Trigger the lint only if there is one nested item
1125 if items.len() != 1 {
1129 // Trigger the lint if the nested item is a non-self single item
1130 let node_name = match items[0].0.kind {
1131 ast::UseTreeKind::Simple(rename, ..) => {
1132 let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
1133 if orig_ident.name == kw::SelfLower {
1136 rename.unwrap_or(orig_ident).name
1138 ast::UseTreeKind::Glob => Symbol::intern("*"),
1139 ast::UseTreeKind::Nested(_) => return,
1142 cx.struct_span_lint(
1143 UNUSED_IMPORT_BRACES,
1145 fluent::lint::unused_import_braces,
1146 |lint| lint.set_arg("node", node_name),
1152 impl EarlyLintPass for UnusedImportBraces {
1153 fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
1154 if let ast::ItemKind::Use(ref use_tree) = item.kind {
1155 self.check_use_tree(cx, use_tree, item);
1161 /// The `unused_allocation` lint detects unnecessary allocations that can
1167 /// #![feature(box_syntax)]
1169 /// let a = (box [1, 2, 3]).len();
1177 /// When a `box` expression is immediately coerced to a reference, then
1178 /// the allocation is unnecessary, and a reference (using `&` or `&mut`)
1179 /// should be used instead to avoid the allocation.
1180 pub(super) UNUSED_ALLOCATION,
1182 "detects unnecessary allocations that can be eliminated"
1185 declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
1187 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
1188 fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
1190 hir::ExprKind::Box(_) => {}
1194 for adj in cx.typeck_results().expr_adjustments(e) {
1195 if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
1196 cx.struct_span_lint(
1200 adjustment::AutoBorrowMutability::Not => fluent::lint::unused_allocation,
1201 adjustment::AutoBorrowMutability::Mut { .. } => {
1202 fluent::lint::unused_allocation_mut